Movement of bacteria-laden waters percolating through fractured bedrock was examined to determine whether effluent originating from conventional waste disposal systems could contaminate shallow ground water supplies. Inoculated waters were injected into holes and/or wells at two geologically different test sites to evaluate the extent of microbial filtration of leachfield effluent in or along bedrock fractures. Injection waters, inoculation with Bacillus stearothermophilis, were found to be readily transported by the ground water gradient into a downslope well. Continued bacteriological analysis showed the tracer bacterium to be present for at least 6 days after inoculation. At mountain building sites, it is essential that either hydrogeologic data, such as bedrock fracture patterns, depth and movement of ground waters, seasonal fluctuations in ground water levels, be fully ascertained prior to installation of waste disposal systems.